The invention specifically relates to the technical field of capacitive rotary position encoders, in which the rotary position is determined with the aid of an overlap—varying with the rotary position—of electrically conductive areas—isolated with respect to one another by a dielectric—and the capacitance value between said areas, which varies in this case. The resulting capacitance value is proportional to the ratio of the area overlap in this case in accordance with a simplifying theoretical model.
In this case, in the rotary encoder, the areas can be rotated one above another contactlessly with an air gap as dielectric, as a result of which no mechanical wear of the sensor system occurs. Besides gases such as air, rotary encoders of this type can also be operated with liquid dielectrics, that is to say for example in water or oil. Alternatively, however, it is also possible to employ areas which are correspondingly insulated with respect to one another by solid dielectrics and which for example also slide on one another.
In order to obtain an absolute rotary position, it is necessary to provide an arrangement of capacitance areas which to enable an unambiguous, that is to say absolute, rotary position determination within the required measurement range, that is to say for example a full revolution. This can be achieved, for instance, by means of a non-rotationally symmetrical configuration of the capacitance ratios. For example with a plurality of capacitive sensitivity areas which are arranged in an annulus and over which sweeps a finger (human or embodied as an electrically conductive electrode) along the annulus, as is known for instance in the case of operating units of portable music players, for instance from US 2005/110768. However, in contrast to such operating units, the rotary position encoders dealt with here have a significantly higher precision and angle measuring accuracy of less than 1 degree, preferably even significantly below that—that is to say for instance in the range of 1/10 degree, 1/100 degree or below that.
Although, given corresponding geometric configuration of the capacitance areas, a linear or known dependence of the capacitance values on the rotary position can be achieved, a position determination of this type is also influenced by diverse disturbing influences (such as e.g. varying area distance, changes in the dielectric (for instance depending on air humidity, temperature), external electric fields, axial offset, non-parallelism of the areas, etc.).
Especially non-rotationally symmetrical arrangements manifest the disadvantage, however, that—especially if an accurate rotary position determination with high angular resolution is required—they are also sensitive to a variety of disturbing influences such as eccentricity, tilting, axial distance variations, wobble, and to external interference fields.
In this case, the “code” asymmetry introduced for the absolute rotary position encoding cannot be differentiated from possible other symmetry shifts as a result of error influences—which leads to measurement errors. In the case of rotationally symmetrical encoding from the prior art, as described for instance in U.S. Pat. No. 5,736,865, such problems can be reduced or avoided in a manner governed by the geometry, but absolute position information is no longer available in this case as a result of the symmetry.
In order to obtain accurate absolute position information, the number of sensitivity areas along the circumference could simply be increased, in order to obtain a higher angular resolution as a result of the smaller angular separation of said sensitivity areas. However, this is complex in practice, both in terms of the production of the position encoder and in terms of the signal evaluation. Standard electronic components for determining capacitance, specifically integrated circuits or IP cores for such circuits, which are manufactured in large numbers and are available in a correspondingly simple and inexpensive fashion, usually provide an evaluation of a limited number of channels, for example approximately up to 10, 12, 14, 16 or 20 channels. Components for evaluating a larger number of channels, for example 50 or more channels, are not available as standard, or require additional hardware outlay by virtue of multiplexer circuits or the like.